STRUCTURE OF ASCIDIANS. 363 



cells form a groove, the medullary groove, the sides of which 

 arch together and form a canal — the medullary canal. Thus 

 the central nervous system is established. The mesoderm 

 seems to arise from outgrowths of the primitive gut or 

 archenteron; the notochord is formed along the dorsal 

 wall of the same, but grows in the tail region only ; two 

 gill-clefts arise from invaginations of ectoderm which meet 

 outgrowths of gut. 



The tadpole-like Ascidian larva enjoys for a brief space a 

 free life. It uses the tail as a swimming organ. But 

 whether it over-fatigues itself, or whether it is under the spell 

 of some constitutional tendency, it soon settles down to 

 sedentary life. It fixes itself by papilla on its head ; the 

 tail shrivels and is absorbed ; the nerve-cord and notochord 

 disappear ; the primary gill-slits are lost in forming the peri- 

 branchial chamber, and the secondary slits replace them ; 

 the whole animal undergoes a metamorphosis, which is one 

 of the most signal instances of degeneration. 



(III.) Salpa and Doliolum Type. 

 Sub-Class Thaliacea. 



These animals are persistently locomotor. They swim in the open 

 sea, like barrels with mouth at one end, exhalent aperture at the other. 

 The cuticle is very thin, as we would expect in animals so active. The 

 muscles form hoops around the body. 



These forms exhibit alternation of generations. In Salpa an asexual 

 generation or "nurse" forms a ventral posterior "stolon." On 

 this, buds are formed which are set free as a chain of sexual Salpa, the 

 links or individuals of which eventually separate. In Doliolum, a small 

 stolon forms a number of primitive buds, which creep over the parent 

 and multiply. They form a lateral series of nutritive individuals, and 

 a median series whose individuals are set free as asexual ' ' foster-mothers. " 

 These carry with them some primitive buds, which divide into secondary 

 buds, which become sexual. Their ova develop into "nurses." 



Classification of Tunicata. 



1. Larvacea. Appendicularia, Oikopleura, Fritillaria, 



Kowalevskia. 



2. Ascidiacea. («) Simple. Ascidia, Phallusia, Ciona. 



(b) Composite. Botryllus, Polydinum. 



(c) Social. Pyrosoma, the phosphores- 



cent fire-flame, 



3. Thaliacea. Salpa, Doliolum. 



